In a system that delivers power to a load, when variable loads are used, such as radiant heaters, incandescent or infrared lamps, or power supplies, a varying parameter such as temperature at the particular load being used will result unless the power delivered to the load is varied in a certain manner to compensate for the parameter variation. When a set value of the parameter at the load is necessary, the need for a power regulation or automatic phase control system becomes apparent.
Accordingly, it is an object of the present invention to regulate the amount of power delivered to a load circuit based on the condition of a predetermined parameter of the load;
It is another object of the present invention to regulate the amount of power delivered to each of the load circuits in a three-phase system based on the condition of a predetermined parameter of the load;
It is still another object of the present invention to maintain equilibrium in the value of a predetermined parameter in the load circuits of a three-phase system by sensing the value of the predetermined parameter of the load and varying the power delivered to the load in accordance therewith.
In order to achieve the above objects of the present invention, a closed loop phase control system for AC Loads has been developed which will assure a "zero-volts" startup and, subsequently, will automatically regulate the amount of power delivered to the load. Although the phase control system can be used with a multi-phase system, it is NOT limited for use with such a system and may be used in conjunction with a single phase system. The initial power turn on occurs always at near "zero voltage" and it starts with a full conduction (180.degree. angle) of the solid state switch (Triac) delivering maximum power to the load. As the sensor output varies, in response to a rising temperature at the load, the conduction angle of the solid state switch will decrease, thus regulating the amount of power required to maintain a given set point temperature at the load. Present technique utilizes a "Time Proportioning ON-OFF Control" which delivers 100% power during the ON period and none during the OFF period, or a manual phase control. The ON and OFF cycles typically last for many cycles of the power source voltage. This low cost circuit combines a "zero voltage switch" at the turn on of the load and a continuous phase control thereafter. The automatic phase control circuit constantly supplies a required amount of load power to maintain thermal equilibrium at a set point. The amount of power delivered to the load can be controlled within each half cycle of the power source voltage. The above technique eliminates ON-OFF switching surges harmful to the filaments and prevents "Overheat" conditions during the ON cycle and "No Heat" during the OFF cycle.